1. Technical Field
This invention relates to a flow measuring apparatus, and, more particularly, to vortex shedding flow meters having sensor means for detecting vortices generated according to fluid flow.
2. Background Art
For many industrial processes, it is common practice to measure a process flow within a pipe using the principle of vortex shedding. Numerous types of vortex flow meters have been designed utilizing this principle to measure the physical characteristics of the fluid flow by sensing the frequency of the vortex formation. Typically, these vortex meters comprise a vortex-shedding body mounted in a flow pipe together with a sensor for detecting the frequency of vortex formation. The sensors used to detect the vortices often contain two highly corrosion resistant process isolation diaphragms which serve to transmit the alternating differential pressure fluctuations caused by the shedding vortices to an internal sensor element through either the sensors internal fill fluid or through mechanical coupling. The fluid flow velocity is proportional to the frequency of vortex shedding.
An example of such a vortex flow meter is The Foxboro Company's E83 series vortex flow meter with optional isolation manifold, described in The Foxboro Company, Product Specification Sheet, PSS 1-8A1 D. This flow meter contains a pipe section containing a shedding plate for generating alternating vortices. A sensor, mounted slightly above the shedder plate, is used for sensing the vortices and generating an electronic signal representative of the shedding frequency. This signal is then transmitted to a processing element contained in an electronic housing for amplification and conditioning of the signal. An isolation manifold is provided for valve control of the fluid communication to the sensors. The isolation manifold is beneficial when the sensor requires maintenance, repair, replacement, or the like, since it disengages the process flow from the sensor without requiring the entire process to be shut off.
A constraint of this type of vortex flow meter is that the measurements depend on the accuracy of one sensor. Sensors do not always operate accurately as they are subject to breakage and malfunctions. In these instances, the measurement of the process flow is interrupted in order to either replace, repair, clean, calibrate, or maintain the sensor. This interruption of the process flow measurement is a limitation on the above vortex flow meter.
It is an object of this invention to provide a vortex flow meter with redundant sensor capability for continuous measurement of a process flow.
It is another object of this invention to provide a vortex flow meter with redundant sensor capability with continuous measurement from a common source point in the process flow.
A more specific object of the invention is to provide dual sensors for use in a vortex flow meter having a process flow such that the sensors enable the process to remain operational despite the malfunction or failure of one of the sensors and to provide a cross check on the computational accuracy of each of the sensors.
A more specific object of the invention is to provide dual sensors for use in a vortex flow meter having isolation valves which can isolate the sensors from the process flow such that either one or both sensors can be replaced without shutting down the process, and without interrupting the measurement of the process flow rate.
A further object of the invention is to provide a cost effective dual sensor vortex flow meter that does not require additional modification to the flow meter body.